(1) Field of the Invention
This invention relates to an inorganic film forming composition. More particularly, the invention relates to a coating composition which forms an incombustible coating film.
(2) Description of the Prior Art
As the coating materials for indoor and outdoor uses, organic coating materials such as aqueous dispersions of vinyl polymers containing almost no organic solvent have been employed for various purposes. However, since they mainly consists of organic materials, they are easily burnt by fire and produce smoke and noxious gases. In order to eliminate the substantial defects of organic coating materials, there has been proposed a method to use inorganic materials as the binders for coating materials. For example, alkali silicate is used as a binder and by subjecting coated films to heat treatment at high temperatures, a coating film having several good properties that are the characteristics of organic compound such as adherence, bonding strength, incombustibility and high hardness can be formed. When the coating film containing alkali silicate is formed by drying at an ordinary temperature, however, the water resistance is not substantially good, so that the coating film is liable to be softened at a high humidity atmosphere or in water, accompanied by the deterioration of a coating film. That is, the alkali silicate contains many alkali metal ions in it and internal siloxane bonds are cut by alkali metal ions, so that, when the coating film formed by drying a coating material containing the alkali silicate as a binder at an ordinary temperature is immersed in water or brought into contact with moisture, the alkali silicate dissolves again and the film becomes soft.
As a material to obviate such defects in the alkali silicate to some extent, though not completely, the colloidal silica aqueous dispersion is known. Within the colloidal silica, siloxane bonds are completely formed and silica particles themselves are completely insoluble in water. When it is dried at a lower temperature, however, the bonds of silica particles-silica particles do not coagulate in the form of the strong chemical bonds of siloxane bonds (.fwdarw.Si--O--Si.rarw.) but coagulate in the form of relatively weak hydrogen bonds such as ##STR1## Accordingly, while the aggregate of colloidal silica once formed by drying is immersed into water for a long time, it disperses as colloidal silica particles in water and the dissociation of the aggregate is caused to occur to some extent. Further, when colloidal silica is solely used for coating, a desirable coating film cannot be formed, since the shrinkage of a coating film is very large and many cracks are caused in the obtained coating film. For this reason, solid contents in the form of powder are added to colloidal silica so as to form good coating films by relieving wet coating films of volumetric shrinkage in drying. For example, as disclosed in U.S. Pat. No. 3,320,082 (W. M. McMahon et al.), zinc rich paint is prepared by adding large amount of metallic zinc powder to colloidal silica and thus prepared coating material is used as an anticorrosive paint for iron and other metals.
Several extenders such as baryte, whiting chalk, silica white, China clay, talc, etc. are generally used as the agents for mitigating volumetric shrinkage of colloidal silica in drying. If a coating film is formed by using such an extender and colloidal silica as a binder, the extender is sufficiently serviceable for the relieving of volumetric shrinkage in drying and a coating film which seems to be desirable at first sight can be formed. It must be noted, however, that the hardness of thus obtained coating film is insufficient, and in addition, the coating films is softened in water and in a high humidity atmosphere as the dissociation of coagulated colloidal silica occurs.